1. Field of the Invention
This invention relates generally to a system for controlling the operation of compressors, and more specifically to a system for controlling the operation of a vibrating compressor at the maximum efficiency with a simple configuration by relating the frequency of an alternating current electric power fed to the vibrating compressor with the temperatures or pressures of a refrigerant sucked and discharged by the compressor.
2. Description of the Prior Art
Heretofore, a refrigerator where refrigeration is effected by using a vibrating compressor to compress a refrigerant gas into a liquid form and causing the liquefied refrigerant gas to evaporate to use the vaporization heat for refrigeration is known. The vibrating compressor used for this purpose is usually divided into the following types; a type using ferrite magnets for maintaining high coersive force, a type using alnico magnets for maintaining high residual magnetic flux density, and type using a combination of ferrite and alnico magnets to take advantage of the benefits of both for improving the magnetic properties of the compressor as a whole.
FIG. 1 shows the construction of the third type of vibrating compressor, which is controlled with the system embodying this invention. In the following, the construction and operation of this type of vibrating compressor.
In a vibrating compressor 500 shown in the figure, a compressor proper 3 is resiliently supported by springs 4 and 5 in an enclosed cylindrical container 2 comprising a cylinder 2a and cover plates 2b and 2c for closing both open ends of the cylinder 2a. A casing 6 of the compressor proper 3 consists of a yoke 7 and a closing member 8. One end of the yoke has such a construction that one end, that is, the upper end of the cylinder 7a is closed with a bottom piece 7b. At the other end of the yoke 7, that is, the lower end of the cylinder 7a, the closing member 8 is installed at the time of assembly. In the casing 6 consisting of the yoke 7 and the closing member 8 provided are two types of permanent magnets; i.e., an alnico magnet and a ferrite magnet, which are disposed at different location, as shown in FIG. 1. The alnico magnet is adapted to be magnetized in the axial direction of the compressor, and the ferrite magnet in the radial direction of the compressor. The length of the alnico magnet in the axial direction of the compressor is adapted to be longer than the axial length of a pole piece 13 formed on an internal iron core 40 so as to ensure uniform magnetic flux in an annular magnetic gap 14. A magnetic path is formed with respect to the permanent magnets 11 and 12 by the cylinder 7a, the bottom piece 7b, the internal iron core 40, and the cylindrical pole piece 13. Within a magnetic gap 14 formed by the cylinder 7a, the bottom piece 7b and the internal iron core 40, disposed is an electromagnetic coil, that is, a drive coil 16, which is vibratably supported by a mechanical vibrating system via resonating springs 20 and 21. A piston 18 is integrally connected to the drive coil 16 via a coil supporting member 17.
An example of the system for controlling the operation of a vibrating compressor noted at the beginning of this Specification is shown in FIG. 2. In FIG. 2, a vibrating compressor 500 is controlled so as to operate in a resonating state, i.e., at the maximum frequency, as a drive power V is applied alternately to the primary windings having different polarities of a transformer 400 by alternately bringing switching transistors TR.sub.1 and TR.sub.2 into conduction. To achieve this, the switching transistor TR.sub.1 and TR.sub.2 are alternately switched into a conducting or non-conducting state in such a fashion as shown by a current waveform in FIG. 3, and the switching frequency is controlled so as to coincide with the resonance frequency of the vibrating compressor 500. More specifically, a base current "I.sub.B " is alternately fed from a drive power source 2000 shown in FIG. 2 to the bases of the switching transistors TR.sub.1 and TR.sub.2 so that a collector current "I.sub.C " shown in FIG. 3 can be switched. That is, a drive power having a desired frequency is obtained as the switching transistors TR.sub.1 and TR.sub.2 are alternately switched into a conducting or non-conducting state by feeding the base current "I.sub.B " having a trapezoidal waveform, as shown by (1) through (3) in the figure, as a current waveform obtained by multiplying "I.sub.B " by a current amplification factor "h.sub.FE " so as to satisfy the condition: EQU I.sub.C .gtoreq.h.sub.FE .times.I.sub.B
at points P.sub.1 through P.sub.3 in the figure. As described above, the conventional type of vibrating compressor 500 has heretofore been operated using a drive power having a frequency coinciding with the resonance frequency of the compressor 500.
In the conventional control method, where the current of the vibrating compressor 500 is controlled so that the switching transistors are switched into a conducting or nonconducting state under the condition I.sub.C .ltoreq.h.sub.FE .times.I.sub.B, the following problems are encountered. Firstly, the signals required for setting the timing for switching the switching transistors into a conducting or non-conducting state are subject to the adverse effects of ripples, leading to fluctuations in the timing of switching. Secondly, since the timing for bringing a switching transistor into a nonconducting state, as shown in FIG. 3, tends to be changed by the current amplification factor "h.sub.FE " for the transistor, the values of the current amplification factor for both the transistors must be agreed with each other. Furthermore, there is another problem of the difficulty in operating the vibrating compressor 500 always at the maximum efficiency due to fluctuation in the current amplification factor "h.sub.FE " due to temperature changes and to secular changes, etc.
To overcome these problems, a system has been conceived, in which the pressures of a refrigerant sucked and discharged by the vibrating compressor 500 are detected, and the frequency of the drive power fed to the vibrating compressor 500 is controlled based on the detected pressures of the refrigerant. This system, however, seems to involve the need for installing the pressure sensors detecting the suction and discharge pressures of the refrigerant on the compressor 500 in a sealed state, leading to complicated construction and increased costs.
FIG. 4 shows a surge voltage suppression circuit of a conventional type used for a car-board refrigerator, which operate a vibrating compressor 500 using a drive power having a frequency coinciding with the resonance frequency of the compressor 500. This circuit has such a circuit configuration as shown in FIG. 4, for protecting the switching transistors TR.sub.1 and TR.sub.2 from surge voltages due to electromagnetic induction in the transformer caused by the alternating actions, i.e., the on-off operation of the switching transistors TR.sub.1 and TR.sub.2. That is, surge voltage absorbing elements, bi-directional varistors 77 and 78, for example, are provided in parallel each across the collector and emitter of each of switching transistors TR.sub.1 and TR.sub.2, which are controlled by outputs Q and Q of a predetermined frequency generated from a drive power generator 2000, and a bidirectional varistor is provided across both ends of a d-c input power source, as shown in FIG. 4. The surge voltage appearing on both ends of the d-c input power source, for example, is absorbed by the varistor 72. Among the surge voltages induced by the electromagnetic induction generated in a transformer 400 by the action of the switching transistors TR.sub.1 and TR.sub.2, the surge voltage induced in the winding 401 of the transformer 400 by the on-off action of the transistor TR.sub.1 is absorbed by the varistor 77 connected in parallel across the collector and emitter of the transistor TR.sub.1, and the surge voltage induced in the winding 402 of the transformer 400 by the on-off action of the transistor TR.sub.2 is absorbed by the varistor 78 connected in parallel across the collector and emitter of the transistor TR.sub.2. In this way, the surge voltage absorbing circuit protects the transistors TR.sub.1 and TR.sub.2 from surge voltages. In addition, as the measure for protecting against excess currents flowing in the transistors TR.sub.1 and TR.sub.2, an overcurrent detecting circuit 74 is provided to detect excess currents to interrupt the outputs Q and Q from the drive power generator 2000. In the figure, numerals 75 and 76 denote diodes, but description on these diodes has been omitted here because they are not directly related to this invention. The windings 401 and 402 of the transformer 400 are wound on the same iron core of the transformer 400.
In the surge voltage absorbing circuit for the conventional type of car-board refrigerators shown in FIG. 4, varistors as surge voltage absorbing elements are provided for each switching transistor. It is desired therefore to reduce the number of parts by protecting two switching transistors from surge voltages with a single varistor.